The electrolysis of aqueous alkali metal halide brines, such as sodium chloride and potassium chloride brines, in an electrolytic cell is a well-known commercial process. Electrolysis of such brines results in the production of products and by-products, that include halogen, hydrogen and alkali metal hydroxide. In the case of sodium chloride brines, the halogen produced is chlorine and the alkali metal hydroxide is sodium hydroxide. The electrolytic cell typically comprises an anolyte compartment containing an anode, and a catholyte compartment containing a cathode assembly. The cathode assembly typically comprises a metallic foraminous cathode and a liquid-permeable (microporous) diaphragm, which diaphragm separates the electrolytic cell into the anolyte and catholyte compartments.
Electrolysis of alkali metal halide brines typically involves charging an aqueous solution of the alkali metal halide salt, e.g., sodium chloride brine, to the anolyte compartment of the cell. The alkali metal halide brine typically contains alkali metal halide, e.g., sodium chloride, in an amount of from 24 to 26 percent by weight. The aqueous brine percolates through the liquid-permeable diaphragm into the catholyte compartment and then is withdrawn from the cell. With the application of direct electric current to the cell, electrolysis of a portion of the alkali metal halide within the cell occurs, and halogen gas, e.g., chlorine, is produced at the anode, while hydrogen gas is produced at the cathode. An aqueous solution of alkali metal hydroxide, e.g., sodium hydroxide, is produced in the catholyte compartment from the combination of alkali metal ions with hydroxyl ions and is withdrawn from the catholyte compartment with the depleted alkali metal halide brine from which the alkali metal hydroxide is subsequently separated.
Historically, asbestos was the most common diaphragm material used in diaphragm cells for the electrolysis of alkali metal brines because of its chemical resistance to the corrosive conditions that exist in such diaphragm cells. Such electrolytic cells have been referred to as chlor-alkali electrolytic diaphragm cells or just chlor-alkali cells. Asbestos in combination with various polymeric resins, particularly fluorocarbon resins (the so-called polymer or resin modified asbestos diaphragms), have been used also as diaphragm materials in such electrolytic cells.
Due in part to possible health and safety issues associated with air-borne asbestos fibers resulting from the use of asbestos in other applications, diaphragms that are substantially free of asbestos, e.g., non-asbestos-containing diaphragms, have been developed recently for use in chlor-alkali diaphragm electrolytic cells. Diaphragms that are substantially free of asbestos, e.g., non-asbestos-containing diaphragms, are often referred to in the art as synthetic diaphragms. Synthetic diaphragms are typically fabricated from fibrous polymeric materials that are resistant to the internal corrosive conditions present in the operating chlor-alkali cell, particularly the corrosive environments found in the anolyte and catholyte compartments.
Typically, microporous (liquid-permeable) diaphragms are formed on the cathode structure by vacuum depositing (in one or more steps) the diaphragm onto the foraminous cathode substrate from aqueous slurries of materials comprising the diaphragm. In the beginning of this depositing process, there is an initial loss of diaphragm materials, e.g., fibrous polymeric materials, that occurs until an inhibiting layer of fibrous diaphragm material forms on the cathode, thereby inhibiting the further passage of solids dispersed in the slurry containing the diaphragm materials through the porous cathode. When the diaphragm is a synthetic diaphragm, this initial loss of diaphragm materials can be costly due to the higher cost of fibrous perhalogenated polymer and halogenated cation exchange polymer materials that typically comprise the synthetic diaphragm, as compared to the cost of asbestos. Further, it is not always possible to re-use the solid diaphragm materials that pass through the porous cathode, which material is referred to as diaphragm slurry filtrate, during the initial formation of the inhibiting layer of the diaphragm. The diaphragm slurry filtrate typically comprises very fine fibers that can blind a foraminous cathode quickly (if used again) and prevent the formation of a diaphragm having the thickness needed to achieve commercially acceptable cell efficiencies. Moreover materials, such as cellulose thickening agents that are present in the diaphragm slurry filtrate are susceptible to being consumed by bacteria found in the slurry.
In order to reduce the initial loss of synthetic diaphragm solid materials during vacuum deposition of the diaphragm, e.g., the inhibiting layer, a preformed nylon net has been placed over the cathode structure before beginning vacuum deposition of the diaphragm. However, it has been found that the fabrication of such a net and its attachment to the foraminous cathode is both time-consuming and expensive, particularly in the case of fingered cathodes. For example, the nylon net is typically cut and sewn into sleeves that are open on one long side. The sleeves can be sewn so as to cover from one to six cathode fingers. Further, each sleeve unit contains additional fabric for the valleys between the fingers of adjacent sleeve units. The sleeves are placed over the cathode fingers with fabric overlapping in the valleys. The periphery of the sleeves is typically attached to the cathode with a glue adhesive.
It has been observed that when a vacuum is applied to the foraminous cathode having a preformed nylon net applied to it during the deposition process, the strands of nylon covering the holes in the cathode can shift and deform, as the nylon can be stretched in one direction, thereby exposing a larger section of the holes in the foraminous cathode than is desirable, which permits the slurry of diaphragm materials to flow more readily through those exposed holes, thereby defeating the purpose of using the nylon net as a partial barrier to prevent excessive loss of diaphragm solids. Further, it has been observed that thin layers of diaphragm material are formed at locations on the cathode structure where the nylon net overlaps or is creased, and diaphragm solids are deposited between the nylon folds. When the nylon disintegrates during cell operation, larger openings are formed in the diaphragm, which openings act as a convenient path for the anolyte to pass through more easily, which in turn causes a reduction in cell efficiency and a possible shortening of diaphragm life.